The present invention relates to an electrostatic copying machine comprising an improved toner image transfer apparatus.
It is known in the art to transfer a toner image from a photoconductive drum to a copy sheet by applying an electrostatic charge to the back of the copy sheet having a polarity which is the same as that of the electrostatic image on the drum and opposite to that of the toner particles. In this manner, the toner particles are attracted away from the drum to the copy sheet. However, this method is not suitable for a color copying system in which three or four toner image transfers are made to a single copy sheet or a multiple copy system in which a number of copies are produced by repeated developing and transfer operations utilizing a single electrostatic image.
Due to the variation of spectral sensitivity of a photoconductor in accordance with the wavelength of incident light, it is frequently necessary to perform exposure and transfer operations for the different primary colors at different surface speeds of a photoconductive drum. It has been found in practice that sufficient control cannot be provided to the transfer operations without an extremely complex apparatus to maintain the color balance within tolerable limits in an apparatus in which an electrostatic potential is applied directly to the backs of copy sheets for toner image transfer.
It has also been determined that this toner image transfer method is not applicable to a multiple copy operation since the electrostatic image is deteriorated at an excessive rate and only a few copies may be produced.
For this reason, it has been proposed to pass the copy sheets between the photoconductive drum and a transfer drum to effect toner image transfer. In this system, the transfer drum is formed with a dielectric peripheral surface and an electrostatic charge is applied to this surface. Such a system allows a much larger number of copies to be produced from a single electrostatic image.
However, it has been determined that, due to the limited photosensitivity of known photoconductors, a toner image transfer operation may be performed more quickly than an imaging operation. In a multiple copying operation, it is therefore advantageous from the viewpoint of copying speed and efficiency to produce the first copy of an original document at a slow speed required for imaging and subsequent copies from the same electrostatic image at a higher speed at which toner image transfer may still be accomplished.
A problem has remained heretofore in practical application in that the electrostatic potential applied to a transfer drum from a charging source such as a corona discharge unit varies as a function of the surface speed of the transfer drum. Specifically, the potential decreases with an increase in transfer drum surface speed since a given area on the surface of the drum spends less time adjacent to the charging unit.
The electrostatic potential on the transfer drum must, however, be maintained at an optimum value for toner image transfer regardless of the surface speed of the transfer drum. If the potential is too high, the electrostatic image on the photoconductive drum will be deteriorated. If the potential is too low, the toner image transfer will be incomplete resulting in copies of insufficient density.